This invention relates to gas turbines and, more particularly, to a concept for efficiently reducing the temperature of air used to cool high temperature turbines in gas turbofan engines.
Modern aircraft gas turbofan engines operate at turbine inlet air temperature levels which are beyond the structural temperature capabilities of high temperature alloys. Hence, engine hot flow path components and, in particular, turbine blades and vanes must be cooled in order to assure their structural integrity in order to meet operating life requirements. It is well understood that gas turbine engine shaft horsepower and specific fuel consumption (which is the rate of fuel consumption per unit of power output) can be improved by increasing turbine inlet temperature. In order to take advantage of this potential performance improvement, modern turbine cooling technology utilizes air-cooled, hollow turbine nozzle vanes and blades to permit operation at inlet gas temperatures in excess of 2000.degree. F. (1094.degree. C.). In general, these sophisticated methods of turbine cooling have utilized compressor discharge or interstage bleed air as a coolant. However, the benefits obtained from sophisticated air-cooling techniques are at least partially offset by the extraction of the necessary cooling air from the propulsive cycle. It can be appreciated that the cooling airflow rate required is a function of the hot gas temperature, increasing with increasing hot gas temperature. Furthermore, the compressor bleed air used for cooling must bypass the combustor and one or more turbine stages, thus giving rise to a performance penalty proportionate to the amount of cooling air utilized. More particularly, the air that is bled from the compressor and used as cooling air for the turbine rotor blades has had work done on it by the compressor. However, because it is normally returned into the flow path gas stream downstream of the turbine nozzle, it does not return its full measure of work to the cycle as it expands through the turbine. Additionally, the reintroduction of cooling air into the hot gas stream produces a loss in gas stream total pressure. This is a result of the momentum mixing losses associated with injecting relatively low total pressure cooling air into a high total pressure gas stream. Thus, the greater the amount of cooling air which is routed through the turbine blades, the greater the losses associated with the coolant become on the propulsive cycle. Thus, while turbine blade cooling has inherent advantages, it also has associated therewith certain inherent disadvantages which are functions of the quantity of cooling air used in cooling the turbine rotor blades.
It will, therefore, be appreciated that engine performance can be increased by reducing the amount of cooling air required by the turbine. Reducing the cooling airflow rate results in improved engine performance with a consequent reduction in specific fuel consumption, the actual magnitude of the cooling airflow rate and specific fuel consumption reductions which can be realized being a function of the specific engine application.
One method of reducing the amount of cooling air required by the turbine is to cool the cooling air entering the hot components. One widely advocated method of cooling the cooling air is to utilize the heat sink capability available in the engine fuel. In such a scheme, the relatively hot cooling air is placed in heat exchange relationship with the relatively cool engine fuel, thereby cooling the cooling air and heating the fuel. The energy extracted by the fuel is reintroduced back into the propulsive cycle as the heated fuel is burned in the combustor, thereby producing what has commonly been referred to as a "regenerative engine". While various studies indicate that fuel-air heat exchangers offer an advantage of small size and low weight, the fuels currently used in aircraft engines (JP4, JP5) are limited in their heat sink capacity, the available heat sink already being used largely to cool the engine oil. To obtain an additional heat sink capacity to permit cooling of the cooling air would require the use of special fuels such as JP7 or JP9, which are currently unavailable in commercial quantities. Additionally, the use of fuel in a fuel-air heat exchanger presents a potential fire hazard which may be unacceptable for commercial engine applications. It will, therefore, be appreciated that another technique for cooling the cooling air is required in order to reduce the coolant flow rate and thereby enhance overall engine performance.